Optimization of an Organic Rankine Cycle Through a Control Strategy for Waste Heat Recovery

Fossil fuel dominates as the power generation source around the globe, and will remain as such in foreseeable future, due to the economic and political difficulties inherent in replacing these–sometimes outdated–technologies with reliable alternatives. In recent years, renewable energy generation has emerged as a competitive market, to the extent that some developed countries aggressively move toward fossil fuel independence (Germany, Denmark, Spain). Energy efficient practices that make renewable generation more economically viable, became of great importance in today’s worldwide situation, especially when considering climate change and air pollution in large cities. The recovery and reuse of heat that otherwise vanishes into the environment appears as one of the viable options. A type of Heat Engine known as the Organic Rankine Cycle (ORC) converts medium-to-low temperature waste heat into electricity. Thanks to technological advances in the past decades, this technique gained competitiveness as an energy efficiency strategy. This work presents the thermodynamic analysis of an ORC working under transient conditions and its response to varying heat input. The ORC utilizes Butane as the working fluid and comprises an evaporator, a turbine, a condenser and a pump. The system was modeled using Matlab Simulink coupled with the software RefProp to obtain the fluid properties at each thermodynamic state. The model assumes that the heat exchangers dominate the transient response, with the rest of the ORC components time scale being negligible in comparison. The system was designed to maintain optimum operating conditions through the use of a Proportional-Integral-Derivative (PID) controller. The control strategys objective is to maintain the system functioning at its highest thermal efficiency against varying input conditions. By modifying the pumps operation (Pressure jump, mass flow rate), we found that the ORC’s thermal efficiency strongly depended on flow conditions entering the turbine. The control strategy maintained optimum thermal efficiency against an unfavorable decrease in the waste heat input.